2021 Sea Surface Temperature
An Exceptional Year
2021 was an exceptionally warm year for the Gulf of Maine. It had the warmest fall on record for the area, and the second warmest summer. Much of the year the region experienced what would be considered “heatwave conditions”. After compiling sea surface temperature data for the entire year we can now conclude that 2021 was the hottest year on Record for the Gulf of Maine.
Departure from Historic Conditions
Thanks to some long-running temperature monitoring programs we are able to track monthly sea surface temperatures as far back as 1850. From this view we can see that recent temperatures are the warmest we’ve ever seen. 2021 setting the new record for the warmest temperature for the region.
Newer Technology: Higher-Quality Data
In 1981 the availability of a higher resolution temperature resource became available in the form of the Optimum Interpolation Sea Surface Temperature (OISSTv2) Satellite Data Record. Once the OISSTv2 data became available we gained access to a higher resolution record, and to the ability to validate past records through comparison. These two data sources now overlap for a period of 42 years and we can see visually that they are not far off:
Because the ERSST data is of a coarser resolution (monthly measurements at a 0.5 x 0.5 degree grid) and does not capture the warmer inshore dynamics (a bias to show colder temperature). The OISSTv2 temperatures in this area are on average warmer. Across the years where data from both sources is available, ERSSTv5 is on average -0.32\(^oC\) or -0.58\(^oF\) Colder than OISSTv2 data.
Warming Relative to the World
While this report focuses on the Gulf of Maine, there is a recognition that this region is not unique in the fact that its temperatures are rising. The Earth as a whole is warming, and that change is being driven by anthropogenic influences to our atmosphere. The impacts of these changes however are not distributed evenly, and there are regions like ours that are warming at rates above the global average.
Warming at an Accelerated Pace
In recent years we’ve seen temperatures in the Gulf of Maine (and adjacent waters) to accelerate rapidly. Scientists believe this is partly due to changes in the behavior of the Gulf Stream Current which brings warm, salty water from the Gulf of Mexico & Florida Northward along the east coast of the US and eventually flowing Northeast past Nova Scotia.
Scientists believe a regime shift has occurred in the Northwest Atlantic as a result of a Northward shift in the Gulf Stream, and a general weakening in the Atlantic Meridional Overturning Circulation Pattern (AMOC). This Northward shift in the Gulf stream does two things: 1) It brings more warm/salty water into the Gulf of Maine, & 2) It obstructs the flow of cold and relatively fresh water from entering the Gulf of Maine from the Labrador current and Scotian Shelf Water (SSW).
These relative contribution of these two impacts may vary through time, but the result (in surface temperature terms) is similar: a warmer Gulf of Maine.
Concerning Temperatures in Recent Years
2021: Daily Temperatures and Heatwave Status
The Gulf of Maine has a seasonal temperature cycle that experiences its lowest temperatures around March, and its peak summer highs in August. The difference between these two seasons is typically 22.91\(^oF\) or 12.73\(^oC\). In 2021 the temperature range was 5.09-20.49\(^oC\) or 15.4\(^oC\), with temperature anomaly values ranging from 0.84-4.09\(^oC\).
The highest temperatures anomalies spiked in the last days of June, capping off an already warm start to the year. A very-warm August and steady, above-average temperatures from early September through the end of the year helped solidify 2021 as the hottest year on record for the Gulf of Maine.
2021: Heatwave Events
During 2021 there were 5 marine heatwave events, lasting a total of 358. These events include one that carried over from the previous year and another that has persisted into 2022. The average duration for these events was 72 days (excluding days in 2020 & 2022).
2021: Record Daily Temperatures
To add context on the severity of these sustained heatwaves, many of the individual daily temperatures throughout 2021 were themselves the hottest days on record.
Of the 365 days in a year, 2021 set a new record temperature on 167 or 45.75% of the year.
Long-Term Heatwave Seasonality
When looking at the last ~40 years of marine heatwave events here in the Gulf of Maine. We see that the frequency, duration, and intensity of heatwave events has increased. This year was no exception with 358 days meeting the criteria for heatwave status.
Temperature Anomaly Horizons
One way to think about the severity of these changes is to think about temperature horizons. A temperature horizon captures how long temperatures remain above certain thresholds. Each threshold is designated its own temperature, and in this way we can see how long within a year temperatures remained: 1 to 2 or as much as 4\(^oC\) above normal.
Comparing the Gulf’s Two Hottest Years
If we pull the horizons of our two hottest years on record it makes it easier to contrast where either one experienced acute high temperature events, and where there were sustained periods of above average temperatures.
Early into 2021 it was apparent that the year was on-par with the previous title-holder for warmest year on record.
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Losing the Balance Between Hot and Cold
Another way to visualize the climate transition that we are observing is by looking at the fraction of each year spent in different temperature ranges. Under a steady climate we would expect over the long-term to spend similar amounts of the year experiencing relatively warm & cold temperatures. These periods would balance themselves out and we would on-average have experienced something similar to the long-term climate.
What we have been seeing in the Gulf of Maine recently has lost that sense of balance. Larger fractions of the year are shared by above average temperatures & cold spell events are becoming vanishingly rare.
Characterizing the Hottest Years
There are a number of ways one might try to describe the extent and severity of temperature extremes. Aside from overall average temperatures there may be acute temperature surges that may have a larger impact than a prolonged warm period. To look more closely at these differences ranked each year on the following characteristics:
- Average overall temperature
- The average temperature above normal
- The cumulative degrees above normal
- The average heatwave length
Mapping Record Temperatures
2021 was an exceptionally warm year for many parts of the World, not just the Gulf of Maine. By taking the average temperature for the year for all years, we can rank how this year compares to previous years in the OISST data. For many places around the world this was one of the top 3 warmest years on record.
Comparing Regions
We are often asked when speaking to the degree of the Gulf of Maine is warming: > “Where is it warming faster?”
This question is challenging because the Gulf of Maine, and any area in the ocean has a unique size/shape/dynamic that makes them difficult to directly compare directly. To put the Gulf of Maine’s warming into perspective against similar ecologically and oceanographically relevant units we can compare it against the large marine ecosystems (LME) of the world.
| Warming Rates of Large Marine Ecosystems | |||
|---|---|---|---|
| Region | Warming Rank | Annual Temperature Change | |
| °C | °F | ||
| Baltic Sea | 1 | 0.047 | 0.085 |
| Gulf Of Maine1 | 2 | 0.044 | 0.080 |
| Black Sea | 3 | 0.044 | 0.079 |
| Scotian Shelf | 4 | 0.041 | 0.073 |
| Iceland Shelf And Sea | 5 | 0.038 | 0.069 |
| Northeast Us Continental Shelf | 6 | 0.038 | 0.068 |
| North Sea | 7 | 0.035 | 0.063 |
| Norwegian Sea | 8 | 0.032 | 0.057 |
| Sea Of Japan | 9 | 0.031 | 0.056 |
| Mediterranean Sea | 10 | 0.030 | 0.055 |
| Data Source: NOAA OISSTv2 Daily Sea Surface Temperature Data. | |||
| Notes: Temperatures adjusted for latitudinal distotion. | |||
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1
Gulf of Maine not a true large marine ecosystem
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Worldwide Temperature Changes
If we look at the rates of change for each grid cell on Earth’s surface, rather than a plot of SST over time, it is possible to rank how each location on Earth is warming relative to others. By ranking those warming rates, and then taking the average ranking across the Gulf of Maine, we can obtain the average warming rank for our area compared to the rest of the globe.
Prior GMRI research found that the Gulf of Maine had been warming at a faster rate than 99% of the world’s oceans. This is still true when comparing warming rates across certain timescales, such as from 2004 to 2013 (when our initial warming study took place).
Over a longer reference period, from 1982 to 2021, the warming rate of the Gulf of Maine, as a whole is still among the highest in the world — increasing at a rate of almost 0.05 °C per year, and faster than 96.2% of the world’s oceans.
However there can be significant spatial variability in this “ranked warming rate” across the Gulf of Maine. Warming rates across the region range from 0.014 °C per year to as high as 0.097 °C per year, meaning some locations over this longer time frame are still warming faster than 99.5% of the world’s oceans.
This variability reflects the various physical oceanographic drivers influencing SST in the region, and in particular the significant interannual variability in the Gulf Stream’s influence on Georges Bank.
Warming Rate Rankings
Based on data from 1982-2021, the warming rates of Gulf of Maine have been some of the highest in the world. The area as a whole has been increasing at a rate of 0.044\(^{\circ}C/year\) which is faster than 95.9% of the world’s oceans.
Over that same period locations within the Gulf of Maine have been warming at rates as low as 0.017\(^{\circ}C/year\) and as rapidly as 0.094\(^{\circ}C/year\), corresponding to ranks as low as 60.3% and as high as greater than or equal to 99.5%.
Fastest Warming Regions
To get a better look at extremes we can mask values below the 80th percentile and look more closely at the differences among the fastest warming areas on Earth. Below only the fastest 20% of locations are shown, to show more detail among the fastest warming areas.
A Note on Data Sources:
NOAA_ERSST_V5 data provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their Web site at https://psl.noaa.gov/data/gridded/data.noaa.ersst.v5.html.
NOAA High Resolution SST data provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their Web site at https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.highres.html.
A work by Adam A. Kemberling
Akemberling@gmri.org